Why Does Java 17 Throw a RejectedExecutionException When Adding Tasks to a ForkJoinPool?

What causes Java 17's ForkJoinPool to throw a RejectedExecutionException when adding tasks?

ForkJoinPool forkJoinPool = new ForkJoinPool();

try {
    forkJoinPool.submit(() -> {
        // Task logic here
    });
} catch (RejectedExecutionException e) {
    System.err.println("Task rejected: " + e.getMessage());
}

In Java 17, a RejectedExecutionException is thrown when attempting to submit tasks to a ForkJoinPool that is unable to accept them. This often occurs when the pool's maximum number of active threads has been reached, leading to task rejection according to its configuration.

// Example of increasing ForkJoinPool's parallelism level
ForkJoinPool forkJoinPool = new ForkJoinPool(16); // Set the parallelism level to 16

// Submitting a task to the pool
forkJoinPool.submit(() -> {
    // Task logic here
});

Causes

• The ForkJoinPool's parallelism level limit has been reached, preventing new tasks from being accepted.
• The pool might be shutting down, hence rejecting any new submitted tasks during its termination phase.
• Tasks are trying to be submitted to a ForkJoinPool that has a saturation (the number of concurrent tasks is maximum).

Solutions

• Check the configuration of the ForkJoinPool, particularly the parallelism level, to ensure it meets your application's concurrency demands.
• Avoid submitting tasks to the pool when it is shutting down by checking its status using methods like isShutdown() or isTerminated().
• Increase the parallelism level of the ForkJoinPool during its creation to allow more concurrent tasks. Example: ForkJoinPool forkJoinPool = new ForkJoinPool(16); // allows more parallel tasks.

Mistake: Assuming the ForkJoinPool works like a regular thread pool without understanding its limitations.

Solution: Familiarize yourself with the differences between ForkJoinPool and other thread pools, especially regarding parallelism and task management.

Mistake: Not checking the status of ForkJoinPool before submitting tasks.

Solution: Always verify if the pool is alive (not shutting down) before submitting tasks to prevent unexpected exceptions.